I did some searching and didn't come up with much..
Probably something like mrsD mentioned or the icons got temporarily scrambled sometimes that happens- not very often
I usually reboot and see if it fixes the problem.

The important thing is that you could still open programs with the start menu.

here is some info on viruses and stand alone scanners from the top AV sites {you might run those scans to double check- these only run when you start it so they don't conflict with your regular av program}
http://neurotalk.psychcentral.com/post403987-4.html